1. Field of the Invention
The present invention relates to a semiconductor device and a method for fabricating the same. More particularly, the present invention relates to a high-voltage metal-oxide-semiconductor (HV-MOS) device and a method for fabricating the same.
2. Description of the Related Art
HV-MOS devices are widely used in power circuits, having particular structures for sustaining high voltages and increasing breakdown voltages. FIG. 1 illustrates the top view of a conventional HV-MOS device, which includes a field oxide (FOX) layer 110, a gate 120, a channel region 130 under the gate 120 surrounded by the FOX layer 110, a source region 140 and a drain region 150 beside the channel region 130 each surrounded by the FOX layer 110, and a drift region 160 between the channel region 130 and each of the source region 140 and the drain region 150. The drift region 160 is formed by implanting a dopant into the substrate using a mask layer having an opening 180 therein as one part of the implantation mask. Before the implantation, the regions predetermined for the channel region 130, the source region 140 and the drain region 150 are covered with another mask layer that defines the active areas and serves as the other part of the implantation mask.
In the above-mentioned HV-MOS device, the FOX layer 110 between the channel region 130 and the source/drain region 140/150 serves as a field isolation layer that allows the device to sustain a high voltage. In addition, the source/drain region 140/150 usually consists of a heavily doped contact region and a lightly doped grade region under the contact region for increasing the breakdown voltage of the device. However, breakdown still occurs easily at the corners of the grade regions.